Question

The following codes crashed:

@interface AppDelegate (PrivateMethods)

@property (nonatomic, strong) NSString * name;

@end

@implementation AppDelegate

- (BOOL) application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

    self.name = @"foobar";
    ...

Error is:

'-[AppDelegate setName:]: unrecognized selector sent to instance 0x6d73df0'

When I change

@interface AppDelegate (PrivateMethods)

to

@interface AppDelegate ()

Then it is okay, what would be the reason?

Update: As answered below, since I must use class extension for this purpose, now this question become: Is the use of class extension to declare private methods acceptable?

e.g.

@interface AppDelegate ()

- (void) start;
@property (nonatomic, strong) NSString * name;

@end

Answer 1

Class extension is basically used to enhance the public property variable. Suppose you have exposed readonly object, or getter method of any variable, then you have make the same object as readwrite in extension.Whereas Category is only used to enhance the method/functionality of class.

check this

@interface MyClass : NSObject
// property here is used as readonly.
@property (retain, readonly) float value;
@end

// Private extension, typically hidden in the main implementation file.
@interface MyClass ()
@property (retain, readwrite) float value;
@end

or

@interface MyClass : NSObject
// here we have exposed getter method of private instance.
- (float) value;
@end

// Private extension, typically hidden in the main implementation file.
@interface MyClass ()
@property (retain, strong) float value;
@end